Acoustic diaphragm and baffle



w. 1.. CARLSON ET AL ACOUSTIC DIAPHRAGM AND BAFFLE Filed Oct. l0, 1947May s, 1951 May 8, 1951 w. CARLSON ET AL ACOUSTIC DIAPHR/acm` AND BAFFLE3 Sheets-Sheet 2 Filed Oct. l0, 1947 May 8, 1951 W. CARLSON ET ALACOUSTIC DIAPHRAGM AND BAFFLE Filed Oct. 10, 1947 DB HELAT/VE JOU/V0RESPONSE 3 Shets-Sheet 5 FOR CI//PVE E FIL TER WIT/'l FIBER PACK/NG /VD46005776 FILTER O/STA/VCE 7'0 MIC. =4FT.

:2.5 @HMS 24 0.5.

| l l l Ja Y fao 2a@ fw@ Z000 TABL HAD/0 JPEA/fE/-P RESPONSE (FREQUENCY)TABLE HAD/a :PEA/ffl? @f5/amfe- W/TH A/vo #wr/mar 6A wry (FREQUENCY)Inventors Patented May 8, 1951 UNITED STATES PATENT OFFICE ACOUSTECDIAPHRAGM AND BAFFLE Wendell L. Carlson and Loy E. Barton, Princeton,

N. 3;, assignors toRadio Corporation of America, a corporation ofDelaware Application ctcber 10, 1947, Serial No. 779,176

8 Claims.- l.

This invention relates to radio receiving apparatus, and moreparticularly to an arrangement for improving the acousticcharacteristics ofthe sound output fromV receivers of small size.

The most common objection to the small, table model'receiver is its poorlow-frequency response. Poor response is due largely to the small barilefor the speaker. which allows the back and iront waves from the speakerto cancel each other. The principal object of this invention is toovercome this detect without increasing the size or interfering with theportability of the receiver.

Those skilled in the art have long recognized the problem of obtaininglow-frequency sound output from dynamic speakers using the very lirnitedbalile area available in small cabinets. It has been proposed, andtried, to obtain a synthetic bass by' accentuating the'harmonics ofthemissing bass notes to give the ear Ythe impression of bass response.Attempts have also been' made to boost existing low frequenciesl bymeans of' selective regenerative feedback in the electrical'v Thesearrangements have' not proven' pension, having a much lower than normalnatural resonance to allow for the stiffness added by the confined airin the small rear enclosure to the compliance of the diaphragmsuspension. It has generally been considered impractical, if notimpossible, to reproduce frequencies lower than the natural resonance oithe speaker, that is, below the point where the speaker diaphragm ismass controlled, in the stiiness controlledre-4 gion. This is trueregardless of the size of the baille, contrary to the generalsupposition that low frequency reproduction is limited only bythe baillearea used.

In accordance with the invention, the lowfrequency response of soundreproducing'apparatus using an ordinary dynamic speaker'is 'ex-v tendedbyenclosing the space to the rear ofthe speaker except for a restrictedopening in the` basel and removably placing the opening overavcorresponding opening in a table top or cabinet' havingW an acoustickdevice, the'back Wave'being In the latter case, it' is fedthroughVthe-openings into said device; They invention is based uponthreeassumptions:y (l) That, inthe home, small table modelreceivers-'areusually located more or less permanentlyI onapiece of iurnituresuch asan end table ;"(2) thatl a special tableor the like could beprovidedespe# cially for thispurpcseand 'so constructed incornbinationwith the' receiver as to increase the efffective bailie areaor otherwisev to increasethere'` model `with ordinary performance.

Another object of the invention is to provide" a small, portableradidreceiver having unusual perform ancer when'used iny onelocation incombination with auxiliary apparatus; which may readily'be disassociatedfrom said auxiliary-'apparatus and transported to other locations foroperation as a complete, small receiverwithatl least usual performance.

Another object ofY the invention is 'to-provide means forobtaining froman ordinary small; table model radio receiver an extended range oflowlfrequencyA sound output comparable' to that ob tained with arelatively large` receiver 'and'withv out using a special speaker-oflower resonant frequency than usual.

A further object of the' invention is to provide an' inexpensivelow-pass acoustic lter forV a radio receiver, preferably using some ofvtl1e"e1ki'st ing apparatusas 'parts of the filter, for'reducing highaudio-frequency disturbances.

Astill further' object of the inventionis 'to providev a supportingtable stand or' cabinet for' a table model radioA receiver, constructedvto im'" prove `the idelityV of the sound output as well as to increasethe radio-frequencyinput to the"a`n`' tenna built within 'the receiver;particularly for operating within shielded' buildings," when the"receiver is used with the' table;

Other objects of the' invention and details of structure Will` be clearfrom the following descripu tion, taken in connection with theaccompanyin'gy drawings, in which:

Fig; 1 is a View in perspective of a radio receiver and table comprisinga preferred 'embodiment of' theinvention, the dotted lines showing thenorrnalr operating positicnof the receiver, and

portions of the table legsbeing cut away'i Fig. 2 is'an elevationalviewpartly in cross'- section', o1" th'e apparatus shown' in Fig.t l', with"the receiver in operating' 'position on the table?" s a 2,552,309" o e 4Fig. :i is a simplified electrical circuit diagram of the apparatusshown in Fig. 1;

Fig. Li is an enlarged view, in perspective, of the chassis of the radioreceiver shown in Fig. 2;

Fig. is a side elevational view, partly in section, of a species of theinvention;

Fig. 6 is a front elevational view of another species of the invention;and

Figs. '7 and 8 are actual characteristic curves of performance taken ofthe apparatus of Figs. 1 and 2.

Referring to Fig. 1, a radio receiver I, made in accordance with the.invention, is shown on top of a table 3, also embodying the invention,the normal operating position of the radio receiver being shown indotted lines I. The table is provided with a hinged lid 5, which maybeclosed when the radio receiver is not used in combination therewith, topresent a substantially smooth table surface. For improving signalreception, a coil of wire 'I is embedded in the surface of the hingedlid and is connected, through the hinges 9, to antenna and ground,respectively, or in some cases to a dipole antenna. When the radioreceiver is placed over the opening I I, for reasons hereinafterdescribed, the loop antenna of the receiver, shown at I3 in Figs. 2 and3, is adjacent the coil 'I for increasing the signal pickup. The latterfeature is particularly desirable in shielded locations where a loopantenna loses its effectiveness and where an outside antenna isnecessary. While only one turn 'I is shown, and while this would operatematerially to increase signal pickup, actually several turns were usedto obtain sufficient inductance to resonate the antenna circuit belowthe broadcast band.

For the purpose of obtaining an increased reproduction of lowaudio-frequency sound waves,

, and materially extending the low-frequency end of the range, aloudspeaker I3, of the moving coil, dynamic type, shown in Fig. 2, isprovided, which has a natural resonance, as measured in a large baffle,substantially below the useful audio frequency range to be reproduced.The loudspeaker includes a direct acting diaphragm I5, and a highlycompliant suspension system for the diaphragm whereby the diaphragm iscontrolled by mass reaction. The loudspeaker is mounted in a closed andacoustically sealed compartment I5 made up of a sheet metal housing Iland a rear enclosure Wall I9, preferably of plywood. The compartment issealed around the rear surface region of the diaphragm I6, except for anopening 2l in the bottom thereof which is arranged to communicate I witha corresponding opening II near the top of the table when the radioreceiver is positioned in a recess 6 above a sub-top 26, as shown. Afelt gasket 25 provides an acoustic seal between the rear compartment I5of the speaker and a closed cavity 30 below the table top formed by thetop 23, bottom 29,- and side walls 2l. In effect, there are twocompartments coupled acoustically through openings II and 2|.

The felt gasket 25 may be cemented to the chassis upon the base aroundopening 2l, as shown in Fig. 4, or it may be cemented to the sub-top`member 25. In the former case, the

compartment I5 is sealed when the receiver is mounted upon any fiatsurface. In the latter case, there is leakage around the lower edge ofthe opening 2l through the Ventilating openings lll (Fig. 4) in thecabinet back 43 when the receiver is operated upon an ordinary flatsurface. The leakage may be increased,V if desired, by spacing thereceiver Vbase above the surface, as by means of a few books or by feet,in either of the two cases, depending upon results desired. Thelow-frequency response changes in the former case as the base opening israised above a flat supporting surface, changing from a sealed, smallback enclosure to a partially open rear enclosure. The felt seal may beentirely omitted, as in Figs. 1 and 2, it having been found thatbeneficial results still are obtained by spacing the openings 2I and I Ia small distance apart.

For the purpose of minimizing resonant peaks and eliminatinganti-resonant dips, shown below 1000 cycles in dotted lines in Fig. 8,the table cavity is filled with sound damping material 3l, such as fibreglass, loosely packed. Other ways of damping known to the art may beused if desirable, for example that shown in Carlisle et al. Patent1,837,755, disclosing coupled compartments but involving a differentacoustic function. The cavity has little or no effect on the responseabove 1000 cycles.

In accordance with the invention, the cubical contents of theV rearspeaker enclosure or comdesired relation, the latter being much greaterthan the former. In an actual working model made in accordance with theinvention, the cubical content-of the region to the rear of the speakerdiaphragm, that is, the enclosed air, was about l0 cubic inches, whereasthe cubical content of the cavity was around 900 cubic inches. Thelatter volume maybe increased for speakers having lower speaker resonantfrequency. The speaker resonance in this case was cycles (Fig. 8), asmeasured in operation with a very large baiiie. The throat opening at 2Iand II bears a certain relation to the cubical contents of thecommunicating chambers and the diaphragm area. The table cavity insidedimensions are 16" x 16" x 31/2, and the table was 23 inches high. Inthe model mentioned, the area of the throat or housing opening was aboutthe same as the diaphragm area and measured Yabout 2.25 x 5".Experiments were made With-the throat opening smaller, and it was foundthat the effect was to lower the low-frequency resonance point, butthere was some loss in amplitude of response. The speaker housingdimensions are, height 5, width 5, and depth 21A". The cabinetdimensions are, height 51A, width lOl/4, and depth 31/2.

Referring to Fig. 4, which shows the invention in greater detail, themetal chassis or plate 32 to which the speaker is secured at points 28,in front of the speaker diaphragm, is provided with a plurality of smallholes 33 of the order of 1/8 in diameter, spaced about 1/2 from eachother for the purpose of providing an acoustic filter in combinationwith a grille 2 (Figs. 1 and 2), preferably also made of metal andhaving holes of similar size and spacing. The diameter of the holes andthe thickness of the metal determine the acoustic inertance.

4bursts, as shown to the'right in Fig. 8. The clot-V ted line curve Cshows the effect of the output response without the filter and the solidline curve'D shows the effect of the lter.

The spaced per-` forate plates provide a low-pass filter for effec- Thelter has little effect upon the low-frequency response .1f the holes areabout the size showngorlarger,

The perforate plate 32 serves also during production to protect theconical paper diaphragm from injury, such as often occurs in cases wherethe diaphragm is unprotected on the chassis during production, and inuse the plate 32 may take the place of a grille cloth.

It will be observed that certain of the radio components are mountedupon the chassis within the speaker compartment and others are mountedon the outside thereof. The tubes 3l and other heat radiating elementsare mounted on the outside of the chassis. The tuning oondenser 39 andother components are also mounted on the outside, but for other reasons.Certain of the connections to the tubes are made on the inside of thechassis, within the compartment, as shown at 4i, for example.

The loop antenna i3 is preferably cemented to an insulating compositionboard dit which forms the rear enclosure wall for the cabinet of thereceiver i. rEhe loop and mounting board d3 are preferably spaced fromthe rear of the chassis to reduce losses. It will be noted that the rearwall of the chassis is made of plywood or other insulating material toreduce losses into the loop circuit.

It will be observed that the closure board 43 is out away at thecorners, at and d8, to provide openings for the purpose of circulatingair to cool the heat radiating elements o-n the chassis within thespaces between the chassis end walls and the cabinet. When the smallreceiver is mounted on an ordinary surface, for example, as shown by thereceiver l in Fig. 1, the air enters the openings at cl at the bottom ofthe cabinet and leaves through openings 4.3 at the top of the cabinet.When, however, the receiver is placed in normal operating position uponthe table, as in Fig. 1 (dotted lines l) and Fig. 2, the lower openingsat 4T! are closed by the proximity to lid 5. Tubular membersS have beenprovided for the entrance of cooling air in place of the coveredopenings at 4l. 'I'he tubular members '.S extend all the way from thetop to the bottom surface of the closed cavity in the table, whilepreserving the acoustic seal within the cavity.

Referring to Fig. 5, an earlier species of the invention, enhancedresults were obtained with an increased baiile eiect between the irontand rear surfaces of the diaphragm, by providing in a cabinet housing 5ian enclosed rear compartment 53 for the speaker, similar to that shownin Fig. 2, arranged to communicate with an opening 55 in a table top 57through a corresponding opening 55S in the bottom of the speakercompartment. As in the other case, the heat radiating elements, such asthe ampliner tubes 6l, were mounted outside of the speaker compartmentfor the purpose of air cooling.

An improvement over the arrangement of Fig. 5 can be obtained by the useof a horn, as shown in the modiiioation of Fig. 6. In this arrangement,a table or cabinet 63 of appropriate ornamental design is provided witha horn 65 extending vertically downwardly from the top S1 to the base ofthe table or cabinet, as shown in dotted lines. A small radio receiverI, with an enclosed speaker, as shown in Figs. 1 and 2, is mounted overthe throat of the horn in the top of the cabinet. This arrangementresults in a substantial extension of the low-frequency end of theoperating range. The horn is not critical in design, although anexponential type is to be preferred. While this arrangement is satthesmall receiver with the bottom opening 2l' closed, as used on anyordinary table or surface. The speaker used for this measurement had anatural resonance of cycles, as determined with a very large baie. CurveB shows the subi stantial improvement in the extended low-frequency endof the range when used with appli. cants invention. In view of theincreased lowfrequency response, it was found desirable to improvecertain of the circuit components, such.

as the output transformer to the speaker, which was redesigned to handlethe increased low frequencies.

Referring to the curves in Fig. 8, curve C in dotted lines shows, in theregion below 1000 cycles, the undesired peaks and dips occurring in thecavity without the sound damping material. The full line curve D shows aslight reduction in the nrst peak and a substantial elimination of thefirst bad dip in curve C. These curves were made with a speaker having1Z0-cycle natural r,eso

nance as measured in a very large baiile, for example, the wall oi aroom. The 10D-cycle speaker used in the case of Fig. 7 was damaged; andit was necessary to obtain another speaker` It had' of the same type tocomplete the tests. a different natural period.

Curve E, the low-frequency portion only being shown, was taken with thesame speaker as in the oase of curves C and D, but operating in Vasingle, large, sealed rear cavity or compartment. In this case, thespeaker was mounted in a hole out in the top wall 23 of the tablecavity, with,

the opening Il closed. The compartment i5 was not involved. A comparisonwith curve D shows that the low-frequency resonance is about This showsthe benecial re-` 25 cycles higher. sults from the two coupledcompartments made in accordance with the invention, as distinguishedfrom a single, large, rear compartment.

From the above, it is apparent that, as a result of the invention,substantially improved results are obtainable with a small, table modelradio receiver using a standard design of speaker when combined with atable or other support which is made to extend the low-frequency endy ofthe range. At the saine time, the small receiver may readily be carriedto other locations and operated in the normal manner as a small radioreceiver with the usual acoustic characteristio output. able, as in thecase of A. C./D. C. battery operation.

In accordance with the invention, a substantial improvement can also berealized when using a small receiver with a sealed rear enclosure andextra-low frequency speaker, as in the above mentioned Olson et al.patent. In one aspect, the invention is an improvement over said Olsonet al. patent. For example, if the speaker, preferably of the foldededge type, disclosed in said Olson et al. patent, has a low naturalresonance of, say, 40 cycles, and is mounted in the sealed enclosure,the stiiness introduced by the small enclosure raises the resonant pointto around 70 The receiver may be fully port-' cycles. Y When such anenclosure, preferably larger than that in the above figures, is providedwith the above disclosed base opening and other features shown in Figs.2 and 4, and mounted upon the table embodying the invention, incommunication with the large cavity, as in Fig. 2, there is much lessstiffness introduced into the diaphragm suspension system and the netresult is a lower resonance, say around 50 to 60 cycles. In such a case,the operation of the small set, with a special low-frequency responsespeaker, operated upon a plane surface with sealed rear enclosure, isbetter than normal as to low-frequency response, but when mounted on thetable and coupled with the cavity according to the invention, theperformance is exceptional. While the main purpose of the invention isto improve the operation of an ordinary small set, with an ordinaryspeaker, the invention is not limited thereto in that substantialimprovement results in applying the invention to a small set with 'aspeaker of special acoustic design.

What we claim is:

1. In a sound reproducing device, the combi nation with a loud speakerhaving an acoustic diaphragm of the direct acting type of a housing forsaid diaphragm, said housing having an acoustically closed compartmenttherein for enclosing the rear surface of said diaphragm and having anopening in one surface thereof for the passage of sound wavesoriginating from said diaphragm rear surface, and a support for saidhousing, said support including a vwalled enclosure defining anacoustically closed cavity cf substantially larger cubical content thansaid cornpartment, one wall of said enclosure having an opening thereinin registry with said housing opening.

2. The invention as set forth in claim 1 wherein damping material isincluded in said acoustically closed cavity for substantially reducingresonance peaks and dips in the frequency response characteristic of thelow-frequency end of the range.

3. The invention as set forth in claim 1 where- J in said housingincludes a walled enclosure for the front surface of said diaphragm, onewall of said enclosure having a plurality of spaced` small holes thereinto provide a low pass filter effect.

4. The invention as set forth in claim 1 Wherein said speaker has anatural resonance, as measured in a large baffle, substantially. belowthe useful audio-frequency range to be reproducedv by said device.

5. The invention as set forth in claim 1 wherehousing independently ofsaid walled enclosure opening for providing circulation of air'in saidhousing.' l f t 7'. `In combination, an acoustic apparatus comi# prisinga table having a top provided with" an opening, a relatively portablesound producing device comprising an acoustic diaphragm of the directacting type, a housing for said device defining around the regionrearwardly of said diaphragm a compartment acoustically closed eX- ceptfor a base opening, and means providing a closed acoustic cavity ofrelatively larger cubical content than said compartment, said meansbeing carried by said tableand coupled withfsaid compartment throughsaid openings for conducting sound Waves from the4 back of saiddiaphragm through said openings to enhance the reproduction of lowfrequency sound waves. 1

8. In a sound reproducing device, the combi-` nation with a loud speakerof the moving coill type having a direct acting diaphragm and aV ofsubstantially larger cubical content than said first-mentionedcompartment, said cavity being acoustically sealed except for an openingdisposed in a wall of said enclosuresaid last-men-j tioned opening beingin registry with said rstmentioned opening whereby the effectiveenclosure for the 'rear of said diaphragm is substantially increased.WENDELL L. CARLSON.

LOY E. BARTON.

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